The invention relates to a device and a method for monitoring a monitored area or region.
Monitoring systems are used in a variety of applications in order to monitor in an automated manner changes in the monitored area. One such application is to protect a hazardous region where the entry of an object into the region is detected and classified according to pre-established rules that establish whether the detection of the object should trigger a warning signal or, for example, turn off a protected machine. This type of access control is predominantly implemented with light grids or laser scanners, but can also be achieved with a camera.
A direct digital recognition of the object is expensive and can be unreliable. For this reason a reflector foil is placed in the monitored area for positively detecting when it is covered and/or obscured by the object. Frequently such a reflector foil is retro-reflective in order to reduce the dependence on ambient lighting.
To detect bright as well as dark objects, the reflector foil is often covered with a pattern of reflecting and non-reflecting areas in order to provide more contrast. For example, dark clothing in front of the reflector and bright clothing or reflective marks on work clothes in front of the non-reflecting background can both be reliably detected.
DE 199 38 639 B4 discloses the use of a variety of reflector patterns in a monitoring system. In order to teach and monitor the position of the reflector patterns, markings are used that have to be recognized automatically by way of a complex definition and placement. In addition, certain manipulations or interferences with the reflector pattern cannot be detected. For example, an area of a shaft which does not have markings cannot be detected. A simple and fast evaluation of the obscurations is not disclosed.
A conventional evaluation of the presence of the reflector allows only a very reduced manipulation detection and provides almost no chance to detect geometric changes in the reflector position. However, above all the evaluation has to be fast and reliable in order to fulfill a protective function.